The invention relates to the field of wafer or substrate plasma reactions, and more specifically to an apparatus and method for exposing a wafer or substrate to a plasma.
In the fabrication of modern integrated circuits, such as microprocessors and memories, oxidation processes are used to passivate or oxidize semiconductor films. Popular methods to oxidize silicon surfaces and films, such as, for example, polysilicon gate electrodes and substrates, include pure oxygen (O2) and water vapor or steam (H22O) oxidation processes. In either case, the oxygen or water vapor is brought into a chamber to react with the silicon-containing surfaces to form silicon dioxide (SiO2).
In many oxidation processes for ultra-high performance integrated circuit applications, a pure SiO2 film may not be desirable as the final structure. For example, although an SiO2 film may provide adequate insulative properties, thin SiO2 films have been found to be penetrable by dopants leading to undesirable results. For example, in complementary metal oxide semiconductor (CMOS) circuits, gate doping is used, in part, to lower the threshold voltage (VT) associated with an individual transistor device. Thus, for example, a polysilicon gate will be doped with boron as part of a PMOS device, or phosphorous, arsenic or antimony as part of an NMOS device. As the gate oxide beneath the polysilicon gate gets smaller, for example in the range of 0.10-0.20 microns or less, dopants implanted into the gate, particularly boron, diffuse or migrate through the gate oxide, particularly during a high temperature annealing activation step conventionally performed to activate the dopants in the diffusion or junction regions. In the case of boron in the gate, some of the boron diffuses through the gate oxide and gets deposited in the channel beneath the gate oxide adding more charge to the channel. The additional charge becomes scattering centers to charge carriers conducting the current. The scattering creates electric field changes that degrade the mobility of the device. The diffusion of the boron into the channel also unacceptably modifies the VT away from the predicted value for a device.
To prevent dopants from diffusing through thin oxides, such as boron through a thin gate oxide, prior art processes have incorporated nitrogen-containing sources such as nitrous oxide (N2O), nitrogen oxide (NO), and ammonia (NH3). The nitrogen-containing material forms a film or layer (typically a silicon nitride (Si3N4) or a silicon oxynitride (SixNyOz) film or layer) that acts as a barrier layer to prevent the diffusion of dopants through the oxide.
In the case of gate oxides, some prior art methods place nitrogen-containing materials or films at the gate oxide/substrate interface. A prior art NO growth and anneal method, for example, typically results in a high nitrogen incorporation (up to nine percent) at a gate oxide/silicon substrate interface. The nitrogen acts as an excellent diffusion barrier, but the nitrogen present in such high percentage at the interface degrades channel mobility. Other prior art methods contaminate the oxide or do not provide a significant penetration barrier to dopants. For example, an NH3 anneal forms a good barrier layer to diffusing dopants, but the reliability of the device is degraded due to hydrogen incorporation. A prior art N2O growth or anneal method incorporates less than three percent nitrogen at the substrate interface, but does not provide a good enough penetration barrier for thin gate oxides.
What is needed is a method and apparatus for incorporating a barrier material on an oxide that may be accomplished in a thermally efficient manner and that does not degrade device performance.
The invention, in one aspect, features a first reaction chamber and a gas source coupled to the first reaction chamber to supply a gas to the first reaction chamber including constituents adapted to react with a substrate in a process step. An excitation energy source is coupled to the first reaction chamber to generate a plasma including ions and radicals from the gas. A second reaction chamber is provided to house a substrate at a site in the second reaction chamber wherein the first reaction chamber is coupled to the second reaction chamber by an inlet member and radicals of the plasma flow through the inlet member into the second reaction chamber. The inlet member includes a passageway having a cross-sectional dimension selected such that during processing the pressure in the second reaction chamber is less than the pressure in the first reaction chamber.
Embodiments of the invention may include one or more of the following features. The second reaction chamber is a rapid thermal processing chamber. The excitation energy source includes a microwave cavity and a microwave generator to provide a microwave field to the microwave cavity. The inlet member includes a main passageway and two passageways which diverge from the main passageway in a direction toward the substrate site in the second reaction chamber. Alternatively, the inlet member includes a main passageway and a face thereof including a plurality of openings. The inlet member passageway is coupled to an output end of the excitation energy source and a sleeve is located in the passageway. The sleeve is made of a material different from the inlet member. For example, the sleeve may be made of silicon, silicon nitride, boron nitride, carbon nitride, or Al2O3.
The invention may further include a rapid thermal processing chamber coupled to the first reaction chamber by a load lock. Additionally, the pressure in the second reaction chamber may be between about 0.50 and 4.0 Torr, while the pressure in the first reaction chamber may be between about 1.0 and 8 Torr.
In another aspect, the invention is directed to an apparatus wherein the inlet member is configured to fit within a preexisting opening in a wall of a second reaction chamber, with radicals of a plasma flowing from a first reaction through the inlet member and into the second reaction chamber.
An interior wall of the second reaction chamber may be curved with a face of the inlet member also curved to substantially correspond to the curvature of the interior wall. An orifice may be provided at an outlet of the first reaction chamber and a cross-sectional dimension of the orifice selected to provide a pressure in the first reaction chamber which is greater than the pressure in the second reaction chamber during processing.
The invention also features, in another aspect, an apparatus for nitridation, including a process chamber in which a substrate can be positioned during processing. A first inlet into the process chamber is provided through which a first process gas can be introduced into the process chamber from a first process gas source. A second process gas source is coupled to a second reaction chamber to supply a second process gas thereto. An excitation energy source is coupled to the reaction chamber to generate a discharge in the second process gas as it flows from the second process gas source through the reaction chamber. An inlet member is coupled between an outlet of the reaction chamber and a second inlet into the process chamber. The inlet member includes a passageway having a cross-sectional dimension selected such that during processing the pressure in the process chamber is less than the pressure in the reaction chamber. The inlet member is configured to fit within a preexisting opening in a wall of the process chamber. The second process gas may comprise nitrogen or a mixture of nitrogen and helium. A valve may be used to selectively provide fluid communication between the first inlet and the first process gas source, and the second inlet and the second process gas source.
In the example of a nitridation reaction in which nitrogen plasma is incorporated into an oxide on the substrate, the nitrogen radicals of a nitrogen plasma rapidly react with the oxide to incorporate nitrogen into the exposed surface of the oxide. In terms of barrier layer protection to a gate oxide, the nitrogen is incorporated primarily in the exposed surface of the oxide and not at the gate oxide/substrate interface. In this manner, the presence of an adequate amount of nitrogen to act as a barrier layer, disposed away from the substrate interface, will reduce the scattering centers caused by otherwise penetrable dopants without deleterious effects on channel mobility.
In another aspect, the invention feature a method for remote plasma nitridation. The method comprises generating a plasma including ions and radicals in a reaction chamber and providing a substrate having an oxide thereon in a rapid thermal processing chamber remote from the reaction chamber. The radicals of the plasma are transferred from the reaction chamber into the rapid thermal process chamber wherein the pressure within the reaction chamber is greater than the pressure in the rapid thermal processing chamber. A portion of the oxide and a portion of the plasma in the rapid thermal processing chamber are reacted and a nitrogen-containing material is formed in a portion of the oxide on the substrate.
In one embodiment, the pressure in the rapid thermal processing chamber is about 0.50 to 4.0 Torr and the pressure in the reaction chamber is about 1.0 to 8.0 Torr.
In another aspect, the invention features a method for remote plasma nitridation including generating a plasma in a reaction chamber from a gas including a mixture of nitrogen and an inert gas. The plasma includes ions and radicals. The radicals of the plasma are transferred into a rapid thermal processing chamber and a portion of an oxide layer on a substrate and a portion of the plasma are reacted to nitrate a portion of the oxide layer.
The inert gas, in one embodiment, may be helium. The gas mixture may comprise no more than about 95 percent helium. Specifically, the gas mixture may comprise between about 20 to 80 percent helium.
In another aspect, the invention features a method for remote plasma nitridation, comprising generating a plasma including ions and radicals in a reaction chamber and providing a substrate having an oxide thereon in a rapid thermal processing chamber remote from the reaction chamber. Radicals of the plasma are transferred into the rapid thermal processing chamber and a portion of the oxide and a portion of the plasma in the rapid thermal processing chamber are reacted at a temperature of between about 800 and 1,100xc2x0 C. for a period between about 60 and 300 seconds to form a nitrogen-containing material in a portion of the oxide on the substrate.
The reacting step, in one embodiment, can take place at a temperature of about 1000xc2x0 C. for about 240 seconds. The step of forming a nitrogen-containing material includes forming one of a silicon nitride and a silicon oxynitride.
In another aspect, the invention features a method comprising positioning a substrate in a rapid thermal processing chamber and introducing a first process gas into the processing chamber through a first gas inlet to deposit a film on the substrate. A second process gas is introduced into a reaction chamber remote from the processing chamber to generate a plasma of the second process gas. The plasma flows from the reaction chamber into the processing chamber through a second gas inlet at a first pressure which is greater than a second pressure in the processing chamber to alter the dielectic properties of a film on the substrate.
In the case of a nitrogen plasma, for example, the method uses a nitrogen plasma to create nitrogen radicals that can be used to incorporate nitrogen into an oxide such as, for example, to act as a barrier layer as described above. The method is useful to incorporate nitrogen into gate oxides and create barrier layers to penetrable gate dopants because, in one embodiment, the incorporated nitrogen does not migrate to the gate oxide/substrate interface. The barrier layer may therefore be created without the deleterious effects on gate performance associated with prior art methods, such as channel mobility degradation.
Additional features and benefits of the invention will become apparent from the detailed description, figures, and claims set forth below.